This invention relates to nuclear reactors and more particularly to a system for transferring fuel assemblies between a nuclear reactor core and a fuel transfer area while the fuel assemblies remain completely submerged in a continuous body of coolant.
During the refueling of sodium cooled fast breeder reactors, it is necessary to remove the decay heat from the irradiated fuel assemblies to prevent their reaching a temperature at which the fuel rod ruptures, because the rupture of the fuel rod results in the release of radioactive gases thus presenting health and contamination problems that are unacceptable. Therefore, it has become accepted practice to provide a means of dissipating the decay heat of an irradiated assembly by providing a system whereby the fuel assembly is kept submerged in a sodium coolant during transfer between the reactor and decay storage.
In most under sodium refueling systems, the means of keeping the fuel assembly submerged in sodium has been to place the fuel assembly in a sodium filled pot, a container with an open end, and then transfer the pot to the desired location. With this type of under sodium refueling system an auxiliary gas or liquid metal cooling system for the pot of sodium is always necessary to meet an emergency condition such as failure of the transfer means. In addition, the ever present possibility of failure of the auxiliary system must be dealt with by providing a backup system. Typically, the cooling capability of the gas system is limited to about 10 KW because of technology limitations. In large commercial fast breeder reactor plants the decay heat from a spent fuel assembly, at the time after reactor shutdown when it is economical to do refueling, is as much as 60 KW or greater. An auxiliary cooling system capable of safely removing that amount of heat, is not presently compatible with the entire reactor plant economics and technology.
In the British Prototype Fast Reactor (PFR) refueling system, the irradiated fuel assembly is stored in a rotor inside the reactor vessel for an initial decay period, and then moved through an opening in the reactor head into a transfer machine located outside of the reactor vessel, an ex-vessel transfer machine. The machine moves above an opening in a transfer tunnel, couples onto a mechanism around the opening, and lowers the fuel assembly through the opening. Equipment within the tunnel moves the fuel assembly laterally in the tunnel to beneath a second opening in the top of the tunnel. A crane mounted machine moves the fuel assembly through a valve on the opening into a fuel handling cell and into further storage prior to partial disassembly for shipping to reprocessing. Aside from the cost problems, there are problems associated with sophisticated interlocks that must be provided to insure against release of fission products during refueling because of the danger to operating personnel. In addition, the valves and adapters associated with coupling the ex-vessel transfer machine to the mechanism around the opening in the reactor head and to the mechanism around the opening in the tunnel are large and expensive. This coupling and uncoupling process is also quite time consuming which increases the refueling time.
The British Commercial Fast Reactor (CFR) refueling system stores the irradiated fuel assembly in a rotor inside the reactor vessel for an initial decay period, then moves it through an opening in the reactor head to a sodium filled compartment and then through an opening in the compartment into a sodium filled compartment outside of containment. The under sodium valves in the openings of the compartments are expensive to install and maintain.
The French Phenix and Phenix 4 refueling systems move the irradiated fuel assembly in a pot through an opening in the reactor head into a gas cooled compartment, then through an opening in the compartment into a decay storage rotor outside of the containment housing the reactor. After a decay period the fuel assembly is moved from the rotor through an opening into a fuel handling cell. In this concept a major safety problem would result if the cooling system of the gas cooled compartment fails and the sodium filled pot containing the fuel assembly becomes stuck in the compartment, as would happen in a power failure. This would result in the fuel assembly overheating and possibly rupturing the fuel rods releasing contaminants.
The Russian liquid metal power reactor refueling systems, in principle, handle the fuel assemblies similarly to the French, and are subject to similar problems.
Other reactor refueling systems have been conceptually designed which keep the fuel submerged in virtually an unlimited amount of sodium. In these concepts the fuel assembly is rotated about a horizontal axis into a horizontal position and then removed from the reactor vessel through an opening in the reactor vessel wall. In this system the sodium level in the ex-vessel transfer machine must be maintained at a higher level than the reactor vessel operating level. Consequently, the loss of this difference in coolant levels could result in loss of cooling to the fuel in transit. Also, the configuration of the horizontal axis machine requires a large lateral dimension significantly increasing the width of the containment building. In addition, the selection of bearings and seals for the horizontal machine is limited because of the constraint that they must operate under sodium.